In recent years, rechargeable electric storage devices including battery cells such as lithium ion battery cells and nickel hydrogen battery cells, and capacitors such as electric double layer capacitors are employed as a power source for vehicles such as cars and motorcycles, and various equipment such as mobile terminals and notebook computers. For example, various types of battery cells are provided. As one of the examples, a battery cell including a flat electrode assembly in which a positive electrode plate and a negative electrode plate are wound in an insulated state from each other, and a current collector connected to the electrode assembly is provided.
As shown in FIG. 15C, the electrode assembly is formed by stacking a separator 23, a negative electrode plate 22, a separator 23, and a positive electrode plate 21 in this order from the inside, winding the stack into a cylindrical shape, and pressing the side surfaces of the cylindrical stack from both sides so as to deform the stack into a flat shape. Alternatively, the electrode assembly is formed by stacking a separator 23, a negative electrode plate 22, a separator 23, and a positive electrode plate 21 in this order from the inside and winding the stack into a flat shape.
As shown in FIG. 15A, the positive electrode plate 21 includes a positive electrode active material layer (positive electrode active material coating) 21b on each of both surfaces of a positive electrode collector substrate 21a formed, for example, by applying a positive electrode active material paint onto one surface of a positive electrode collector substrate 21a composed of a strip-shaped aluminum foil, followed by drying, and thereafter applying the same positive electrode active material paint onto the opposite surface of the positive electrode collector substrate 21a, followed by drying.
As shown in FIG. 15B, the negative electrode plate 22 includes a negative electrode active material layer (negative electrode active material coating) 22b on each of both surfaces of a negative electrode collector substrate 22a formed, for example, by applying a negative electrode active material paint onto one surface of a negative electrode collector substrate 22a composed of a strip-shaped copper foil, followed by drying, and thereafter applying the same negative electrode active material paint onto the opposite surface of the negative electrode collector substrate 22a, followed by drying.
More specifically, the positive electrode plate 21 includes the positive electrode active material layer 21b on each of both surfaces of the positive electrode collector substrate 21a excluding one end portion in the width direction, for example, by applying the positive electrode active material paint onto each of both surfaces excluding the one end portion of the positive electrode collector substrate 21a. Therefore, the one end portion serves as a portion where the positive electrode collector substrate 21a is exposed (positive electrode active material layer-unformed portion 21c). On the other hand, the negative electrode plate 22 includes the negative electrode active material layer 22b on each of both surfaces of the negative electrode collector substrate 22a excluding one end portion in the width direction, for example, by applying the negative electrode active material paint onto each of both surfaces excluding the one end portion of the negative electrode collector substrate 22a. Therefore, the one end portion serves as a portion where the negative electrode collector substrate 22a is exposed (negative electrode active material layer-unformed portion 22c).
As shown in FIG. 15C, each separator 23 serves to physically isolate the positive electrode plate 21 and the negative electrode plate 22 from each other, and to retain an electrolyte.
It should be noted that the negative electrode active material layer 22b is coated to a width larger than that of the positive electrode active material layer 21b in consideration of deposition of dendrite, etc. Further, the separator 23 has a width larger than that of the positive electrode active material layer 21b and the negative electrode active material layer 22b for securing the insulation. However, the separator 23 has a width so as not to cover the positive electrode active material layer-unformed portion 21c and the negative electrode active material layer-unformed portion 22c that project in the width direction.
An electrode assembly 20 is formed by winding the positive electrode plate 21, the negative electrode plate 22, and the separators 23. At this time, the positive electrode plate 21 and the negative electrode plate 22 are shifted from each other to the left and right in the width direction, thereby allowing the positive electrode active material layer-unformed portion 21c in a stacked state to project from a lateral end of the negative electrode plate 22 on one end side of the electrode assembly 20, and allowing the negative electrode active material layer-unformed portion 22c in a stacked state to project from a lateral end of the positive electrode plate 21 on the other end side of the electrode assembly 20, as shown in FIG. 16A. Thus, the electrode assembly 20 has positive electrode projections 20a on one end side, and has negative electrode projections 20a on the other end side.
Further, as shown in FIG. 16B, each of the positive electrode projections 20a has a bound portion 20b formed, before the projection 20a is bonded to a current collector 40, by bringing pieces of the projection 20a on the distal end side into tight contact with each other thereby binding them together, and a sloping portion 20c sloping from the proximal end side of the projection 20a toward the bound portion 20b. Similarly, each of the negative electrode projections 20a of the electrode assembly 20 also has a bound portion 20b formed, before the projection 20a is bonded to a current collector 40, by bringing pieces of the projection 20a on the distal end side into tight contact with each other thereby binding them together, and a sloping portion 20c sloping from the proximal end side of the projection 20a toward the bound portion 20b. 
As shown in FIG. 16C, the positive electrode current collector 40, for example, composed of aluminum or aluminum alloy is arranged on one surface of the bound portion 20b of the positive electrode projection 20a, and a backing plate 50 similarly composed of aluminum or aluminum alloy is arranged on the other surface of the bound portion 20b thereof. The current collector 40 and the backing plate 50 are bonded to each other, for example, by ultrasonic bonding together with the bound portion 20b. The negative electrode current collector 40, for example, composed of copper or copper alloy is arranged on one surface of the bound portion 20b of the negative electrode projection 20a, and a backing plate 50 similarly composed of copper or copper alloy is arranged on the other surface of the bound portion 20b thereof. The current collector 40 and the backing plate 50 are bonded to each other, for example, by ultrasonic bonding together with the bound portion 20b. 
Meanwhile, in the sloping portions 20c of the projections 20a, the positive electrode active material layers 21b are not formed on the positive electrode collector substrates 21a, or the negative electrode active material layers 22b are not formed on the negative electrode collector substrates 22a. Therefore, a gap is formed between adjacent positive electrode active material layer-unformed portions 21c, or between adjacent negative electrode active material layer-unformed portions 22c. Therefore, the sloping portions 20c are comparatively susceptible to compressive deformation due to the force received from the bound portions 20b. 
Accordingly, for example, in a battery cell mounted on a car, if vibration occurs during traveling, the current collectors 40 vibrate at a different amplitude or frequency from that of the electrode assembly 20, which may possibly cause damage to the sloping portions 20c due to the edges of the backing plates 50 rubbing or abutting the sloping portions 20c. Damage to the sloping portions 20c tends to cause defects such as a decrease in the current collecting function, and an increase in internal resistance of the battery cell.